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NASA Technical Memorandum 100855

Optically Interconnected Phased Arrays

[NASA-Ta- 10085 5) OPTICALLY I: NTESCOL~Y~CTL~~ N 8 8 - 2 '1 4 00 PHASED ARRAYS (NASA) 9 p CSCL d3A Uriclas G3/33 0 136320

Kul B. Bhasin and &chard R. Ku~th Lewis Research Center Cleveland, Ohio

Prepared for 1 Advances in and Superconductors: Physics and Device Applications sponsored by the Society of Photo-Optical Instmmentation Engineers Newport, California, March 13-18, 1988

5 Optically interconnected phased arrays Kul B. Bhasin and Richard R. Kunath National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio 44135

ABSTRACT Phased-array antennas are required for many of NASA's future missions. They will pro- vide agile electronic beam forming for and tracking in the range of 1 to 100 GHz. Such phased arrays are expected to use several hundred GaAs monolithic integrated circuits (MMIC's) as transmitting and receiving elements. However, the inter- LnCO connections of these elements by conventional coaxial cables and add weight, 0 0. reduce flexibility, and increase electrical interference. Alternative I w based on optical , optical processing, and holography are under evaluation as possi- ble solutions. In this paper, current status of these techniques will be described. Since high-frequency optical components such as photodetectors, , and modulators are key elements in these interconnections, their performance and limitations are discussed.

1. INTRODUCTION Phased-array antennas based on GaAs monolithic microwave integrated circuits (MMIC's) are of interest as the next-generation systems for communications and tracking applications in space missions. These missions include Mars Rover (Mars-to-Earth data relay), Mission Planet Earth (Earth radiometry with active array ), and the Experimental Antenna System in advanced communication satellites. Phased arrays having hundreds of elements are becoming a reality with the maturity of GaAs MMIC . However, the current signal distribution methods (such as waveguides and coaxial cables) for phase and amplitude control signals and radiofrequency (RF) carrier signals to GaAs MMIC radiating elements suffer from high weight, mechanical inflexibility, and high loss. Optical techniques have the potential to overcome these 1imitations.l For example, opti- cal as a is lightweight, small in physical size, mechanically flexible, highly resistant to electromagnetic interference (EM11 and electromagnetic pulses (EMP), and virtually without loss. Its loss is compared with media in Figure 1. There are several approaches to optically controlled phased-array antennas, but most of them fall into two categories: one is based on the use of fiber and high- frequency optical components for signal distribution in phased arrays, and the other is based on the use of advanced optical processing techniques to achieve phase and amplitude changes in the microwave beam in phased-array antennas. Both of these approaches are dis- cussed in detail in this paper. From the components point of view, in the past several years, GaAs substrates have pro- vided the basis for the development of monolithic microwave technology. Low-loss microstrip lines can be fabricated on this semi-insulating substrate. The high saturation velocity provides the essential microwave device technology. At the same time optical waveguides, lasers, and detectors can be fabricated on a GaAs substrate with convenient control of the composition and thickness of GaAlAs layers. The feasibil- ity of fabricating a microwave and an optical integrated circuit on the same substrate pro- vides an opportunity for monolithic integration of both optical and microwave functions in optically controlled phased-array antenna applications in space communications to provide low weight and reduced complexity in systems. The status of these components is discussed in the following section. 2. ARCHITECTURES FOR OPTICALLY INTERCONNECTED PHASED ARRAYS The integration of MMIC's into microwave phased-array communications systems has .iden- tified optical interconnects as an enabling technology (Fig. 2). Two areas of development that are critical to the successful implementation of fiber are architectures and integratable high-frequency optical components and devices on GaAs substrates. An analysis of potential architectures reveals that, at present, all of the possible architec- tures can be represented by following basic types for phase shifting from 0" to 360". Other architectures are possible where 360' phase shift is not required. 2.1 Fiber-optic-based architecture Control signal distribution is the least demanding architecture in terms of device technology development .2 The typical configurations assume a central control unit from which element assignments are determined and transmitted (Fig. 3). Addressing schemes of this type result in element-by-element control signal distribution, which requires high serial bit rates to address large numbers of elements in demanding communications traffic scenarios.3 An alternative design concept is to provide each element in the array with a minimal amount of processing capability, thereby distributing the system intelligence and easing addressing hardware constraints. The lower frequency requirements of control signal architectures (<1 GHz) allow for a more timely development. However, many technology issues are common to all fiber architec- tures. These features include the integration of optical electronic integrated circuits with GaAs MMIC designs, fiber device , and device efficiency. The RF signal distribution architectures are generally characterized by the point at which the communications data and the RF carrier are combined. In CPU-level designs, the communications data are impressed on the RF carrier before conversion to light. This type of architecture is the simplest conceptually, but is the most demanding on the optical requirements of the link, such as , dynamic range, and noise figure. A more realizable architecture at present is that of combining the communications data with the RF carrier at the transmit or receive MMIC module. The greatly reduced high- frequency RF bandwidths required lead to relaxed system specifications, but two fibers are needed instead of one, and components for mixing RF and data must be incorporated in the MMIC design. 2.2 Optical-processor-based architecture A second type of architecture does not use controllable MMIC's as the devices necessary to create the appropriate phase and amplitude excitations for antenna beam formation.4n5 Rather, this function is implemented in the optical domain (Fig. 4). Optical beam forming network (OBFN) architectures use holographic techniques to coinbine spatial and temporal information for antenna beam formation. By optically creating the desired antenna beam characteristics, the difficulty in producing nearly identical MMIC phase shifters and variable gain amplifiers is overcome. The OBFN architectures have temporal device requirements similar to other fiber architectures, but have unique spatial device requirements. Temporal , photode- tection, and high-power light generation are common to all RF signal distribution architec- tures. Additionally, spatial modulation, spatial sampling, and optical signal combining devices are needed to completely develop OBFN architectures. 2.3 Holoqraphy-based architecture Free space optical interconnections usin? a holographic optical element have been proposed for high-speed electronic circuits. The feasibility of free space optical inter- connection technique has not been fully assessed as applied to optically interconnected phased arrays. 3. OPTICAL MODULATION TECHNIQUES While spatial modulation is unique to OBFN architectures, temporal modulation is common to all fiber architectures. Temporal modulation can be characterized by direct, indirect, and injection-locking techniques (Fig. 5). Injection locking differs from the other two techniques in that it does not directly support the RF carrier, but is used as a reference to synchronize free-running microwave oscillators at the desired frequency. Conceptually, direct modulation is a straightforward technique.7 The RF communications signal is used to directly modulate a diode. However, the demands for a directly modulated link are significant. As with other microwave/millimeter transmis- sion systems, direct links require large dynamic ranges, high center frequencies, wide bandwidths, and low noise levels. At present, modulation frequencies are limited to 30 to 40 GHz, largely because of the relaxation oscillation effects of laser diodes. Relative noise (RIN) effects, the primary noise source in optical links, result in modest bandwidths at reduced dynamic ranges. At frequencies above 8 GHz, direct modulation technique specifications such as threshold current and depth of modulation are usually traded off to achieve very reasonable link performance.

2 Indirect modulation techniques use external devices such as Mach-Zehnder interferome- ters together with laser sources to modulate optical carriers. The interferometric-type devices use phase modulation between the legs of the interferometer to generate an intensity-modulated optical signal. Most indirect modulation techniques use devices that operate at 1300 nm on substrates, like LiNbO3, that until recently were incompatible with GaAs processing techniques. In interferometric devices, the RF communications signal is used to modulate the of one leg of the interferometer, which phase modu- lates the laser light and results in an intensity-modulated optical signal. Indirect modu- lators suffer from high insertion losses and therefore require significant optical power from the laser source. However, to achieve low noise in the optical carrier it is desira- ble to operate the laser at low power. The comparison of directly modulated and externally modulated RF fiber-optic links is performed by Stephens et a1.8 Injection-locking techniques are the third major catagory of temporal modulators. This type of modulator differs from the others in that it supports the RF carrier indirectly. Optical injection locking is used to feed a synchronizing RF reference signal either directly or indirectly to a free-running microwave oscillator. The indirect tech- nique uses a low-power continuous wave (CW) RF signal to modulate a laser source, and this ' optical synchronization signal is transmitted via fiber, detected and amplified, and fed to a free-running microwave oscillator.9 The direct technique is similar except that the optical synchronization signal is applied directly to a photosensitive MESFET microwave oscillator without the need of predetection and amplification. The advantage of both tech- niques is the relaxed optical power constraint, and the synchronizing reference signal may be a subharmonic of the microwave oscillator frequency and use the nonlinear operational mode of the laser to generate the correct harmonic for injection locking. At present, injection locking is the only temporal modulation technique to be usable beyond 20 GHz. 4. SYSTEM COMPONENTS The feasibility of several system architectures discussed in section 2 depends on the development of several key high-frequency optical components. The current capability of these components in terms of frequency or speed of operation is shown in Figure 6. More detailed discussions for each component are presented in the following subsections. 4.1 Optical inteqrated circuits In an active, solid-state phased array based on a fiber-optic network, an optical fiber from the central processing unit will be connected to the MMIC module for the phase and gain control functions. The RF input to the MMIC's will be connected to the baseband processor by an optical fiber, if feasible. It may be possible with to combine the two links on a single fiber. Implementing various optical fiber links for an MMIC phased-array signal distribution network will require integrated optical and receivers on GaAs substrates for 0.8- to 0.9-pm transmission. The MMIC transmit and receive modules with optical integrated feed circuitry are shown in conceptual diagrams (Figs. 7 and 8, respectively). Interfaces for phase and amplitude control of the MMIC receiver and MMIC require transmission of the digiti1 signal by optical fiber. The input signal to the transmitter, the local oscillator signal to the receiver, and the IF output from the receiver will require RF-optical links. A variety of optical electronic integrated cir- cuits (OEIC) have been demonstrated,1° although their compatibility with MMIC fabrication processes has yet to be determined. The NASA Lewis Research Center has taken the initiative to develop a photoreceiver on a GaAs substrate that will control the phase and gain functions of an MMIC. A GaAs pho- toreceiver circuit operating up to 1 Gbit/sec and also employing a 1:16 demultiplex chip on a GaAs substrate tested with a clock speed up to 2.7 GHz has been demonstrated.ll These operations are now being fully integrated to demonstrate the phase and amplitude control of GaAs MMIC's. The OEIC's required for the RF signal interface require high bandwidth photodetectors. The status of such photodetectors is discussed in subsection 4.3. The direct optical con- trol of phase shifting and gain functions of MMIC's is also a possibility that can further simplify the MMIC/optical interface. Optical control of microwave devices and circuits has been demonstrated;12i13 however such techniques can provide switching but not suffi- cient phase shifting, and monolithic integration of such methods also has to be shown. 4.2 Laser sources As discussed in section 3, the RF signal can be upconverted to an optical frequency by directly modulating the current and the modulated signal detected after

3 transmission over a fiber-optic link.l4 Fully packaged commercial GaAlAs/GaAs can operate up to 10 GHz when biased to maximum output power. For GaInAs/InP lasers operating at 1.3 pm, modulation response up to 18 GHz has been demonstrated.15 The direct modulation bandwidth of these lasers is limited to the relaxation oscillation frequency. Recently, in highly p-doped GaAs/GaAlAs multiple quantum Well lasers, a relaxation oscillation fre- quency of up to 30 GHz has been observed.l6 Direct modulation of the laser offers the advantages of small size, low loss, and ease of operation. Relative intensity noise, nonlinearity, and power consumption remain major problem areas. 4.3 Photodetectors Photodetectors in discrete form as well as integratable with GaAs MMIC's are required to demodulate microwave signals carried by an optical fiber up to 100 GHz. The optical of 0.82 to 0.84 and 1.3 to 1.5 pm are of interest. Photoconductors with interdigitated surface geometrics and fabricated on GaAs MESFET-like structures have been demonstrated up to 10-GHz bandwidths, and their perform- ance in optical receivers has been evaluated.l7 Similiar photoconductors have also been studied based on heterostructures . l8 The gain bandwidth product in these photoconductors is limited by spacing and saturated velocity of in the GaAs or heteros- tructure layers. A semi-transparent Schottky barrier designed and fabricated by Wang et a1.19 has been operated in excess of 20 GHz. By further reducing the active layer geometry to a 5-pm square, bandwidths have been demonstrated in excess of 100 GHz.2 At 1.3 pm, GaInAs PIN have been operated up to 30 GHz.~~ Ease of integration with GaAs MMIC's, low noise, and high quantum efficiency remain the major criteria for the selection of photodetectors for phased-array applications. 4.4 Modulators Indirect modulation techniques require the use of external modulators which take advan- tage of the electro-optic and electron absorption phenomena. So far, modulators based on electro-optics effects, such as Mach-Zehnder interferometric techniques, have shown band- widths u to 17 GHz in LiNbo3 substrates21 and 20 GHz in GaAs substrates at 1.3-pm wave- length.25 The advantage of GaAs is obvious, since it will allow integration with laser sources, detectors, and GaAs digital electronic circuitry. The major advantages of modulation are low noise and separate optical and microwave signals. Additional weight, optical damage thresholds, and insertion loss remain major disadvantages. 5. CONCLUSIONS Optical techniques have the potential to provide light weight and flexible interconnec- tions for the GaAs MMIC phased-array antenna. As array element numbers and frequencies get higher, optical beam forming networks will become a necessity. However, at present, system tradeoffs are not clear, and only detailed architectures studies will point out when and what type of optical techniques will be applied to phased-array antennas. Parallel developments in high-frequency optical components will assure early breadboarding of optically interconnected phased arrays. 6. REFERENCES

1. A.M. Levine, "Fiber Optics for Radar and Data Systems," in Laser and Fiber Optics Communications, SPIE Vol. 150, M. Ross, ed., pp. 185-192, SPIE, Bellingham, WA (1978). 2. J. Austin, and J.R. Forrest, "Design Concepts for Active Phased-Array Modules," IEE Proc. Part F: Commun. Radar Signal Process., vol. 27, pp, 290-300 (1980). 3. K.B. Bhasin, and G. Anzic, R.R. Kunath, and D.J. Connolly, "Optical Techniques to Feed and Control GaAs MMIC Modules for Phased Array Antenna Applications," in 11th Communi- cations Satellite Systems Conference, pp. 506-513, AIAA, New York (1986). (Also NASA TM-87218). 4. G.A. Koepf, "Optical Processor for Phased Array Antenna Beam Formation," in Optical Technoloqy for Microwave Applications, SPIE Vol. 477, S.K. YaO, ed., pp. 75-81, SPIE, Bellingham, WA (1984).

4 5. L.P. Anderson, F. Boldissar, D.C.D. Chang, "Antenna Beamforming Using Optical Process- ing," to be published in SPIE Vol. 886, 1988.

. 6. L. Bergman, A. Johnston, R. Nixon, S. Esener, C. Guest, P. Yu, T. Drabik, M. Feldman, and S.H. Lee, "Applications and Design Considerations for Optical Interconnects in VLSI," in Optical Computinq, SPIE Vol. 625, J.A. Neff, ed., pp. 117-126, SPIE, Bellingham, WA (1986). 7. H.W. Yen, C.M. Gee, and H. Blauvelt, "High-speed Optical Modulation Techniques," in Optical Technology for Microwave Applications 11, SPIE Vol. 545, S.K. Yao, ed., pp. 2-9, SPIE, Bellingham, VA (1985). 8. W.E. Stephens, and T.R. Joseph, "System Characteristics of Direct Modulated and Externally Modulated RF Fiber-optic Links," J. Liqhtwave Technol., vol. LT-5, pp. 380-387 (1987). 9. P.R. Herczfeld, A.S. Daryoush, A. Rosen, A.K. Sharma, and V.M. Contarino, "Indirect Subharmonic Optical Injection Locking of a Millimeter-Wave IMPATT Oscillator," IEEE Trans. Microwave Theory Tech., Vol. MTT-34, pp. 1371-1376, (1986). 10. J.K. Carney, M. Helix, and R.M. Kolbas, "Operation of Monolithic Laser/MultiDlexer Optoelectric IC," in Optical Interfaces fo; Diqital Circuits and Systems, SPiE Vol. 466, R. A. Milano, ed., pp. 52-58, SPIE, Bellingham, WA (1984). 11. G. Gustafson, M. Bendett, J. Carney, R. Mactaggart, S. Palmquist, F. Sc-hit, K. Tan, and w. Wolters, "GaAs Circuits for Monolithic Optical Controller," to be published in SPIE Proceedings Vol. 886. Los Angeles, CA, Jan. 1988. 12. R.G. Hunsperger, "Optical Control of Microwave Devices," in Inteqrated Optical Circuit Enqineerinq 11, SPIE Vol. 578, S. Sriram, ed., pp. 40-45, SPIE, Bellingham, WA (1985). 13. R.N. Simons and K.B. Bhasin, "Microwave Performance of an Optical-ly Controlled AlGaAs/ GaAs High Electron Mobility Transistor and GaAs MESFET," in 1987 IEEE MTT-S Interna- tional Microwave Symposium Diqest, Vol. 2, pp. 815-818, IEEE, Piscataway, NJ (1987). 14. K.Y. Lau, C. Harder, and A. Yariv, "Direct Modulation of Lasers at f >10GHz by Low- Operation," Appl. Phys. Lett., Vol. 44, pp. 273-275 (1984). 15. C.B. Su, V. Lanzisera, R. Olshansky, W. Powazinik, E. Meland, J. Schlafer. and R. B. Lauer, "15 GHz Direct Modulation Bandwidth of Vapour-Phase Regrown 1.3 mm InGaAsP Bur- ied-Heterostructure Lasers Under CW Operation at Room Temperature," Electron Lett., Vol. 21, pp. 577-579 (1985). 16. K. Uomi, T. Mishirna, and N. Chinone, "Ultrahigh Relaxation Oscillation Frequency (up to 30GHz) of Highly p-doped GaAs/GaAlAs Multiple Quantum Well Lasers," Appl. Phys. Lett., Vol. 51, pp. 78-80 (1987).

17. s.J. Wojtczuk, J.M. Ballantyne, Y.K. Chen, and S. Wanuga, "Monolithically Integrated Photoreceiver with Large Gain-Bandwidth Product," Electron. Lett., Vol. 23, pp. 574-576 (1987). 18. T. Umeda, Y. Cho, and A. Shibatomi, "Picosecond HEMT Photodetector," Jpn. J. Appl. Phys. Lett., Vol. 25, pp, L801-LEO3 (1986). 19. S.Y. Wang, D.M. Bloom, and D.M. Collins, "20-GHz Bandwidth GaAs Photodiode," Appl. Phys. Lett., Vol. 42, pp. 190-192 (1983). 20. J.E. Bowers, C.A. Barrus, and R.J. McCoy, "InGaAs PIN Photodetectors With Modulation Response to Millimetre Wavelengths," Electron. Lett., Vol. 21, pp. 812-814 (1985). 21. C.M. Gee, G.D. Thurmond, and H.W. Yen, "17-GHz Bandwidth Electro-optic Modulator," Appl. Phys. Lett., Vol. 43, pp. 998-1000 (1983). 22. S.Y. Wang, S.H. Lin, and Y.M. Houng, "GaAs Travelling-Wave Electro-optic Modulator with Bandwidth in Excess of 20 GHz at 1.3 pm," Appl. Phys. Lett., Vol. 51, pp. 83-85 (1987).

5 100 5 WLITUDE OPTICAL 2 CONTROL7 CWONENTS L E: 10 MICROWAVE 4 3r WAVEGU I DE 2 4c 1 R F-, F-, . MICROSTRIP LOW-LOSS OPTICAL FIBER '-7 / ' '1'1h1 ' Ir'1'1' ' IrlrkI OPT I CAL .. .1 ' 1 ' 10 ' MODULATED mrc FIBER FREQUENCY, GHZ DATA ARRAY FIGURE 1. - CWARISON OF OF MICRO- FIGURE 2. - GENERIC ARCHITECTURE FOR OPTIC-BASED PHASED WAVE TRANSMISSION LINES WITH OPTICAL FIBER. ARRAY.

'I. OPTIC

II FOURIER 11 RADIATING // TRANSFORM -' ,! ELEMENT IMAGE BEAnJ rREFERENCE BEAM

8 rBEAM

- i ELECTRO-OPTIC MO'DULATOR I RF INPUT MIC MODULE FIGURE 3. - OPTICALLY CONTROLLED AND FED WNOLITHIC INTE- FIGURE 4. - OPTICALLY PROCESSED BEAM FORMING NETWORK. GRATED CIRCUITS FOR PHASED ARRAY ANTENNA.

100

80 UNDER RESEARCH AND DEVELOPPENT CJ COmERCIALLY 2 60 RF INPUT^^) b W AVAILABLE > (A) DIRECT MODULATION. Yw

Ws E 40

RF INPUT (B) INDIRECT MODULATION. 20

fO 0 PHOTO- LASERS (C) SUBHARMONIC INJECTION LOCKING. DIODES CONDUCTORS FIGURE 5. - VARIOUS SCHEES FOR TRANSMISSION OF MICROWAVE FIGURE 6. - PERFORMANCE OF CURRENT SOLID-STATE SIGNAL VIA OPTICAL FIBER. OPTICAL CWONENTS CWATIBLE WITH MMlC INTE- GRATION TECHNIQUES.

6 GAAs LASER

CONTROL OIC

I TTL DRIVERS I 1 I

RF 01C MATCH I NG RECEIVE FEED ELEMENT OPT I CAL o> NODULATED OPTICAL RF SIGNAL ‘HIGH-FREQUENCY GAAS LASER dc BIAS INPUT RECEIVE WlC

MATCHING

OPTICAL LOCAL DETECTOR OSCILLATOR SIGNAL LOCAL OSCILLATOR FIGURE 7. - BLOO: DIAGRM OF RECEIVE MIC WITH OPTICAL INTERCONNECTS.

OPTICAL FIBER GAAs LASER

CONTROL RF OSCILLATOR OIC

MODULATOR TTL DRIVERS I I I I J L 1 RF OIC -- /- OPTICAL FIBER MATCHI NG Gas LASER NETWORK

POWER AMPLIFIER TO RADIATING ELEMENT

~~~~ ~~ TRANSMIT MMIC FIGURE 8. - BLOCK DIAGRM OF TRANSMIT MIC WITH OPTICAL INTERCONNECTS.

7 Natimal AeronauI8CS and Report Documentation Page Space Adrnmostratmn 1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. NASA TM-100855 4. Title and Subtitle 5. Report Date

Optically Interconnected Phased Arrays 6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No. Kul B. Bhasin and Richard R. Kunath E-4058

10. Work Unit No. 506-44-20

National Aeronautics and Space Administration 14. Sponsoring Agency Code Washington, D.C. 20546-0001

I 15. Supplementary Notes 1 Prepared for Advances in Semiconductors and Superconductors: Physics and Device Applications sponsored by the Society of Photo-Optical Instrumentation Engineers, Newport, California, March 13-18, 1988.

16. Abstract Phased-array antennas are required for many of NASA's future missions. They will provide agile electronic beam forming for communications and tracking in the range of 1 to 100 GHz. Such phased arrays are expected to use several hun- dred GaAs monolithic microwave integrated circuits (MMIC's) as transmitting and receiving elements. However, the interconnections of these elements by conven- tional coaxial cables and waveguides add weight, reduce flexibility, and increase electrical interference. Alternative interconnections based on optical fibers, optical processing, and holography are under evaluation as possible solutions. In this paper, current status of these techniques will be described. Since high- frequency optical components such as photodetectors, lasers, and modulators are key elements in these interconnections, their performance and limitations are discussed.

17. Key Words (Suggested by Aulhor(s)) 18. Distribution Statement I Phased arrays Unclassified - Unlimited Optical control Subject Category 33 i Antenna studies

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages 22. Price' Uncl assi f i ed Unclassified 8 A02